Visible and IR light receptors are capable of detecting electromagnetic radiation (light) at various intensity levels and at various wavelengths (colors) in the spectral region from approximately 400 nm to 5000 nm. Examples of such light receptors include but are not limited to the human eye and optical detectors/sensors which produce a response (thermal, electrical etc.) whenever illuminated.
The human eye, optical detectors/sensors and photo-receptors can be damaged by exposure to high intensities of light. For example, optical detectors can be exposed beyond their capabilities and destroyed by either continuous or short duration exposure to a laser beam. Similarly, the retina of the eye can be damaged by being exposed to a laser beam for only a brief period of time. In effect, the retina and the nerves within the retina are burned by the intense light of the laser beam.
In laboratory and industrial settings, protective glasses or goggles are commonly worn to protect the eyes from exposure to laser light. These eye protection devices are based on filtering techniques and, typically, are made from tinted or colored materials which may be selected for protection over specific wavelength ranges of laser light. Because of the coloration of these materials, light at wavelengths other than that of the laser light are attenuated. This restricts abroader portion of the spectrum, significantly altering the spectral features of the transmitted light. As a result, these protective glasses or shields typically have a disadvantage that the visual perception of the colors of objects and images observed are either altered or obscured. For example, goggles suitable for protection against green laser light at 532 nm, a frequency doubled Nd:YAG emission, or at 514.5 nm, an argon ion laser emission, are typically orange colored. This does not allow the wearer to perceive colors in the blue or green region of the visible spectrum.
While not all environments of usage require unaltered spectral transmission, there are numerous environments which have strict requirements for substantially unaltered spectral transmission. One specific example of such an environment is within an airplane cockpit. Many aircraft, both military and civilian, are using color coded instrumentation to display information for the pilot. Additionally, light displays projected on the wind screen or canopy and known as "heads-up" displays are color images which, if optically blocked, filtered or altered, may not be filly visible to the pilot.
The usage of lasers capable of blinding the cockpit crew of an aircraft has become a hazard for both military and commercial air traffic. It is therefore necessary to protect pilots' eyes from exposure to intense laser radiation. It is not feasible to block the transmission of all wavelengths of light commonly emitted by lasers using existing filtering techniques inasmuch as the ability of the pilot to observe both instrumentation inside the cockpit and other objects outside the cockpit would be so altered as to render the flight crew partially blinded or severely limited.
Photo-reactive optical limiters such as sun glasses that change color with higher visible light spectrum intensity are commercially available. The reaction time of the photo-reactive elements is quite slow, on the order of 1-90 seconds, and is definitely not fast enough to provide any protection from laser beams which may be of a pulsed or continuous wave type. Thus, a laser beam impinging on the eye through an eyeglass lens of this type will not be attenuated sufficiently or quickly enough to prevent damage to the eye.
A protective device capable of protecting the human eye or optical detectors from laser radiation must have a very fast photo-reactive response and yet must be of sufficient transparency in the spectral region applicable to a given light receptor to allow it to function. In the case of visual perception, the protective device to be effective must also permit, without distortion, observation of color images as advantageously used in certain environments requiring protection from exposure to laser radiation.